The present invention relates to a method and a device for monitoring the functioning of a compressor, in particular a compressor used for pneumatic load-leveling of a vehicle body.
East German Patent Application No. 219 535 A1 discloses monitoring a pressure prevailing in the working chamber of a compressor and comparing the pressure to a predefined setpoint pressure, a deviation in the actual pressure from the setpoint pressure leading to an error signal. The actual pressure is recorded using a measuring element, and is compared in a comparator unit to the stored setpoint value determined in a previous acceptance test. The time characteristic of the actual pressure, combined with other performance quantities, provides evidence of system malfunctions. Functional errors occurring in the components of the device are able to be localized, for example, on the basis of a change in the accumulator pressure, combined with temperature information.
The setpoint value is provided as a static pressure value, not, however, as a curve profile that is variable over time. For that reason, reliable evidence of malfunctions can only be provided in a steady-state operation of the device, when the pressure in the accumulator chamber of the compressor deviates by a predetermined measure from the setpoint pressure. Dynamic fluctuations, which are not attributed to malfunctions, but rather to system-controlled processes, such as actuation or de-energizing, can be misinterpreted by the comparator unit and lead to erroneous information.
To rule out such erroneous information, the system must be monitored virtually continuously and, to compensate for the fluctuations, an average value of the actual pressure must be generated over long operating phases, and then compared to the adjusted setpoint value. This, in turn, increases the memory and computational outlay required for the comparator unit.
Additional system information can only be utilized if relevant comparison criteria, tailored to the specific system information, have been formulated and stored in advance in the comparator unit. However, this increases the outlay required for supplying, storing, and calculating data. Substantial outlay is required to make concrete inferences about the location and the nature of an error that has occurred, otherwise reliable information on malfunctions cannot be provided.
An object of the present invention is to devise a reliable method for monitoring a compressor using minimal outlay.
The present invention provides a method for monitoring the functioning of a compressor, in particular a compressor which is used for the pneumatic load-leveling of a vehicle body and which applies a working pressure to a gas-filled accumulator chamber (3). The actual pressure (Pactual) prevailing in the accumulator chamber (3) is measured and compared to a setpoint pressure (Psetpoint), and an error signal is produced in response to a deviation in the actual pressure (Pactual) from the setpoint pressure (Psetpoint). The comparison of the actual pressure (Pactual) and the setpoint pressure (Psetpoint) is carried out during the accumulator charging operation, i.e. when the accumulator chamber is charged.
The present invention also provides a device or monitoring the functioning of a compressor, in particular for implementing the method of the present invention. The device comprises a pressure sensor (5) for measuring the actual pressure (Pactual) of a gas-filled accumulator chamber (3), which is able to be charged via the compressor (2), and comprises a control unit (6), which is able to be supplied with the actual pressure (Pactual) in the form of an input signal, which is able to be compared in the control unit (6) to a setpoint signal corresponding to a setpoint pressure (Psetpoint). An error signal is able to be produced in response to a deviation that exceeds a preset tolerance value (Ptol). A control signal generated by the control unit (6) is able to be fed to the compressor (2) for charging the accumulator chamber (3) at the same time as the measurement of the actual pressure (Pactual) is performed. The setpoint-actual comparison is carried out by the device.
The accumulator chamber to be charged by the compressor advantageously supplies pneumatic functional elements, in particular the air bellows of an air spring in a motor vehicle. To obtain reliable information about the status of the compressor, it suffices in accordance with the present method that a comparison of the actual and setpoint values be performed simply during the accumulator-chamber charging processes, it being necessary to carry out these processes at regular intervals anyway. On the other hand, the need for continuously recording the operating state of the compressor and of the accumulator chamber during regular operation is eliminated, thus substantially reducing the quantity of data to be evaluated. Moreover, any fluctuations that occur during regular operation no longer play a part, so that there is no need to perform statistical compensating calculations to smooth out fluctuations in the actual pressure by generating mean values over long periods.
The characteristic curve of the setpoint pressure is known as a function of time for the entire accumulator charging process, so that the measured actual pressure can be compared continuously or quasi-continuously to the setpoint pressure during the charging operation. The setpoint pressure curve also includes the information on the change in the accumulator pressure over time, so that an error signal is produced in response to a pressure rise that deviates from the momentary setpoint value.
A tolerance band, within which no error signal is produced, is placed advantageously around the pressure setpoint curve. The nature of an error can be inferred from the deviation in the actual pressure above or below the tolerance band. If the actual pressure is underneath the tolerance band during the charging operation, i.e, the predefined pressure cannot be reached, then this suggests a leakage in the accumulator chamber, in the supply lines, discharge lines or in a valve, or to a defective compressor. If the actual pressure is above the tolerance band during the charging operation, then this suggests a blocked line or a valve that is not opening.
In accordance with the device of the present invention, the actual pressure of the accumulator chamber is recorded using a pressure sensor and fed as an input signal to a control unit, where the input signal is compared to a setpoint signal indicative of the setpoint pressure. This takes place contemporaneously with the compressor charging the accumulator chamber. Accordingly, the accumulator charging operation and the measuring process, inclusive of the evaluation, are coupled to one another via the hardware, so that tests can be performed at regular intervals, and the amount of data to be evaluated will nevertheless remain within limits.
The pressure in the accumulator chamber rises non-linearly; as accumulator pressure increases, the characteristic pressure curve becomes flatter. The characteristic setpoint pressure curve can be expressed as a function of time as a second-degree polynomial, the three coefficients of the polynomial being advantageously uniquely defined on the basis of a reference device, which includes a reference compressor with a corresponding accumulator chamber, by measuring three pressure/time measuring pairs during one unique accumulator charging process, and then being stored in the control unit. This polynomial describing the characteristic setpoint pressure can be used as a reference for compressors of a similar design.
The non-linear rise in the characteristic pressure curve necessitates that only the same setpoint segments and actual-pressure segments be compared during the accumulator charging process. Since the pressure drop in the accumulator chamber is a function of a lower pressure value from the particular sampled pressure, the lower pressure value is to be considered as the initial pressure for the setpoint/actual comparison, by effectively expressing the setpoint pressure as a function of the initial pressure and of the differential pressure. The differential pressure of the characteristic setpoint-pressure curve can be expressed from transformations from the second-degree polynomial as a function of the initial pressure and of the time difference that has elapsed between the measurement of the initial pressure and the measurement of the instantaneous actual pressure. Through this transformation, one selects only that segment of interest from the entire characteristic setpoint-pressure curve, assuming an initial pressure that changes with every new operation, and compares this segment to the corresponding actual values.